FIG. 1 illustrates a sound transmitting apparatus 14 disclosed in U.S. Pat. No. 6,782,106 B1. The sound transmitting apparatus 14 includes an external sound receiver 20 (such as a microphone), a volume controller 22, and a mixer 28. The external sound receiver 20 is for receiving an external sound (ambient sound). The volume controller 22 is for controlling an input sound when necessary. The mixer 28 is for mixing a reproduced sound signal from a sound producing device and an external sound signal from the volume controller 22 to provide an output. The volume controller 22 only outputs the reproduced and external sound signals whose volumes exceed a predetermined value. In other words, external or reproduced sounds whose volumes are small will not be transmitted to a user.
When a pedestrian walks on a street, the pedestrian can listen to music using a music player having the sound transmitting apparatus 14 of U.S. Pat. No. 6,782,106 B1. Therefore, the music player can be used to alert the pedestrian whenever any loud sound occurs on the street.
However, a loud sound is not necessarily a sound that represents danger and that should be noticed by a pedestrian. When a pedestrian can realize the type and direction of a loud sound through other ways (such as by sight), the pedestrian does not have to rely on his or her hearing to respond to the loud sound.
That is to say, when a pedestrian wears earphones on both ears listening to music, the pedestrian can rely on his or her sight to notice an abnormal sound or a sound representing an emergency situation that comes from the front, and does not need to use the sound transmitting apparatus 14 of U.S. Pat. No. 6,782,106 B1 to output the sound coming from the front as a notification to the pedestrian. However, it is necessary to output abnormal sounds or sounds representing emergency situations that come from behind the pedestrian through the earphones so as to notify the pedestrian. Therefore, there is a need to find a solution such that a pedestrian who is wearing earphones to listen to music does not hear sounds coming from the front but is still able to hear abnormal sounds coming from behind through the earphones.